This invention relates to a dry etching method applied to the manufacture of semiconductor devices. More particularly, it relates to a method for selective anisotropic etching of a stacked system of a compound semiconductor not containing aluminum and a compound semiconductor containing aluminum, as typified by a GaAs/AlGaAs stacked system.
A monolithic micro-wave IC (MMIC), produced by stacking GaAsMes-FETs (metal semiconductor field effect transistors) into an integrated circuit, is superior in response to high speed and high frequency and in low power consumption and hence has become popular as devices for communication between moving objects or satellite communication.
There has also been known a high electron mobility transistor (HEMT) which has also realized the response to a higher speed and a higher frequency of the GaAsMes --FET. The HEMT takes advantage of the properties of a two-dimensional electron gas of the GaAs compound semiconductor of being able to travel at a fast speed on the hetero junction interface without being scattered by impurities. For implementing the high degree of integration with the HEMT, the necessity arises for a dry etching technique which renders it possible to perform etching with higher accuracy and higher selectivity.
Above all, during the process of selectively etching the GaAs/AlGaAs stacked system for forming a gate recess, it is crucial that the etching be performed with high precision and a high selectivity ratio. The reason may be summarized as follows. In a hereto-junction FET, such as HEMT or hereto MIS structure FET, the threshold voltage is determined by the thickness or the concentration of impurities of the lower AlGaAs layer. Thus the concentration of impurities and the thickness of the AlGaAS layer are pre-set depending on the desired threshold voltage. However, during the process of removing the upper GaAs layer for forming the gate recess, if the selectivity ratio for etching is insufficient such that over-etching or the like is caused, the thickness of the AlGaAS layer tends to be changed, with the consequence that the threshold voltage of the resulting hereto-junction FET tends to be deviated from the desired threshold voltage.
The selective etching of the GaAs layer on the AlGaAs layer is performed by reactive ion etching (RIE) using a gas mixture composed of a CFC gas and a rare gas. Thus an example of employing a mixed gas of CCL.sub.2 F.sub.2 /He is described in Japanese Journal of Applied Physics, Vol. 20, No. 11 (1981), p. L847 to 850. With the reactive ion etching, making use of the mixed gas of CCL.sub.2 F.sub.2 /He, the GaAs layer is removed by Ga mainly forming a chloride and by As forming a fluoride and chloride. When etching reaches the underlying AlGaAs layer, AlF.sub.x (aluminum fluoride) having a low vapor pressure is produced on the exposed surface to terminate or decelerate the etching. In the above-mentioned publication, disclosing such technique, the selectivity ratio is reported as being approximately 200. On the other hand, the anisotropic shape of the gate recess is maintained because the sidewall surface of the pattern is protected by the deposition of a carbonaceous polymer derived from the decomposition product of the CCl.sub.2 F.sub.2 gas or from the resist mask.
Meanwhile, if CCl.sub.2 F.sub.2 is released to an external environment, there arises the risk of destruction of an ozone layer in the stratosphere. It is therefore preferred that the use of such CCl.sub.2 F.sub.2 be avoided as far as possible in view of protection of the earth's environment, and hence it is preferred that the use of the CFC based gases such as CCl.sub.2 F.sub.2 be avoided as far as possible.
In the dry etching employed for fabrication of semiconductor devices, it has been proposed to use so-called substitution freon having a shorter life in the stratosphere, such as hydro chlorofluoro carbon (HCFC), in place of the above-mentioned CFC gas.
However, the substitution freon causes a problem similar to that with CFC based gas because of the organic halogen compound making up the substitution freon. This problem consists in the tendency towards increase in particle contamination due to the carbonaceous polymer generated in a gas phase under discharge dissociation conditions. It is difficult to control the generated amount of the carbonaceous polymer, which is deposited on the resist mask surface or on the pattern sidewall surface to contribute to improved resist selectivity and shape anisotropy, such that, if a fine pattern of a high density is to be formed, the disc substrate tends to be destroyed by shorting.
If the carbonaceous polymer is to be employed for achieving sufficient selectivity and anisotropy for establishing a method for producing a semiconductor device not employing an organic halogen compound gas, one has to resort to a resist mask as a supply source for the carbonaceous polymer. However, if the resist mask is sputtered with ions having a high incident energy, with a view to supplying a necessary amount of the carbonaceous polymer, the resist selectivity tends to be lowered to produce a critical dimensional loss, while it is also not possible to prevent damages to the underlying layer or generation of particle contamination.
For achieving protection of the wafer surface while preventing particle contamination and suppressing etching damages, it is effective to intensify the properties of the membrane of the carbonaceous polymer itself to permit a sufficiently high wafer surface protective effect to be displayed despite a decreased amount of polymer deposition and to substitute another material unlikely to become a source of contamination for a part of the carbonaceous material for avoiding the dependency solely on the inhibitive effect of the carbonaceous polymer.